A full moon spectacular (because it is at its perigee)

The moon appeared at its largest and brightest for 15 years yesterday. Above, the moon as seen for Winterberg in central Germany. Photograph: Gernd Thissen / EPA

A night-time spectacle not seen for 15 years was produced last night when a full moon coincided with the moon orbiting the Earth almost 18,400 miles (30,000km) closer than usual this month.

The moon's orbit is not a circle but an ellipse, and when it is at its closest approach, or perigee, the moon comes much closer to the Earth than at its apogee, or greatest distance, hence its apparent swelling.

Despite the fact that it was still around 221,800 miles away, the rare conjunction meant that the moon appeared much larger and brighter than it usually does.

Or, as the blog of the Royal Observatory Greenwich noted a little more scientifically: "On average, the Moon is 378,000 km away, and at furthest, it is 399,300 km away from the Earth's surface. So [the] full Moon is 6% closer than average, and so 11% brighter than average! (Or, to put it another way, it is 11% bigger & 20% brighter than when the Moon is at its furthest point away from us)."

Nasa estimated that the moon could appear up to 14% bigger and 30% brighter than other full moons this year.

The closeness of the moon last night also had an effect on tides. Thanks to its increased proximity, it exerted an even greater pull than the usual full moon. The resultant perigean tide drags the high-water mark up another 5cm (2in).

However, the Environment Agency said last night that it was not expecting the unusual tides to cause any significant problems around the British coastline.

"The risk of tidal flooding is still low," said a spokesman. "But there are 31 flood watches in place. This is to do with rain rather than any lunar effects." Fifteen watches were in place in the north-west of England, nine in the Midlands, and seven in the Thames area.

Yesterday's celestial show was also slated to kick off with the "moon illusion" - when the moon appears to swell as it lies close to the horizon - before segueing into the annual Geminid meteor shower. The Geminids, which rank among the finest shooting star displays, streak through the sky at a comparatively slow 21.75 miles a second, with as many as 100 meteors shooting past every hour.

The Geminids stream travels in the wake of the gigantic, mountain-sized asteroid Phaethon, which was discovered in October 1993 by the Infrared Astronomy Satellite. It measures 3.2 miles in diameter, and takes 18 months to orbit the Sun.

It is probably the extinct hulk of a comet's nucleus which released the Geminids meteoroids during its active phase some 4,000 or more years ago. The Geminids' splendour, though, might have been slightly dimmed by the huge, bright moon.

The next big event on the astronomical calendar is the winter solstice, which takes place on December 21.

Those who missed the spectacle last night should take heart - it is scheduled again on November 14 2016.

• This article was amended on Wednesday 17 December 2008. The metric figure was wrong when we said a perigean tide "drags the high-water mark up another 50cm (2in)". We meant to say 5cm. This has been corrected.

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